The present application pertains to integrated circuit amplifier circuits, and more particularly, to line drivers with controlled output impedance.
When utilizing a line driver for sending signals over transmission lines, it is often necessary to match the characteristic impedance of the line in order to avoid reflections and ringing. Transmission line impedances in the range of 50 to 200 Ohms are commonly used. Additionally, transformers with different turn ratios are often needed which results in a wider range of reflected line impedances to be matched by the line driver. Various approaches are used to match the impedance of the line driver and the transmission line.
One approach matches the impedance by driving the line with either an ideally zero impedance amplifier in series with a resistor or an ideally infinite impedance amplifier in parallel with a resistor. This approach has the disadvantage of a substantial increase in the power consumption due to power dissipated by the added resistor. The alternative is to design an amplifier with a finite output impedance equal to the characteristic impedance of the transmission line. To be able to use such an amplifier with different transmission lines or transformers, its output impedance should be programmable. Thus, it would be desirable to have a programmable output impedance amplifier.
One technique for implementing a controlled impedance driver utilizes an amplifier with voltage and current feedbacks as shown in FIGS. 1(a) and 1(b) and disclosed in U.S. Pat. No. 5,121,080 by Scott and Swanson, the disclosure of which is expressly incorporated herein by reference. An amplifier 200 with two current outputs is used. The current from the first output 210 is set proportional to the second output 220. The first output 210 is coupled to the inverting input of the amplifier, providing the current feedback path. The second output 220 provides the amplifier's output and is coupled to the inverting input of the amplifier by a feedback conductance g.sub.f. By proper selection of the feedback resistance and the current feedback ratio, the gain and the output impedance are set.
The output impedance in Scott and Swanson is a function of the current ratio as well as the feedback resistor value and therefore this approach utilizes a constant ratio between the current feedback and the output current. In order for the feedback current to be a constant proportion of the output current either high output impedance devices should be used for both outputs so that its Ids is not a function of Vds, or the Vds of the two output devices 230 and 240 must track. Long channel length devices have larger output impedance and could be used in devices 230 and 240, but the longer channel length increases gate capacitance and also uses more silicon area. Cascocling is a common technique for buffering the drain of a current mirror from voltage variations, but the stacking of devices limits the amount of output signal swing and again increases silicon area. To force the two output voltages to track, a resistor R.sub.d was placed in the current feedback path (as shown in FIG. 1(b)) so that output current variations create similar variations in the two output voltages. While this approach is adequate for the signals driven by the amplifier, if a reflected signal from the line side reaches the amplifier's second output the resulting change in current will force the voltage of the first output in the opposite direction. If in the presence of reflected signals the two voltages do not adequately track, a change in the ratio of feedback current to output current may result, which in turn results in a change in output impedance.
Further this single ended controlled impedance driver may not be able to adequately tolerate reflections from the line when driving outputs near zero. For example, if the amount of current needed to maintain a constant output impedance in presence of reflections is greater and opposite polarity compared to the current flowing in the output of the amplifier, the amplifier may turn off completely.
Another complication resulting from the added resistor R.sub.d is that the resister makes programmability of the output impedance more difficult. The output impedance is a function of the resistor R.sub.d and for different line impedances a different value of R.sub.d should be used. To match different line impedances, different transformers were used to create the characteristic impedance reflected to the driver. However, the disadvantage of this approach is the need for hardware modification.
Thus, it would be desirable to design an amplifier with controlled output impedance even in the presence of reflected signals. Furthermore, it is desirable that such an amplifier be easily programmed for various line impedances without having to make hardware changes.